Electronic devices comprising active organic materials are attracting increasing attention for use in devices such as organic light emitting diodes (OLEDs), organic photoresponsive devices (in particular organic photovoltaic devices and organic photosensors), organic transistors and memory array devices. Devices comprising organic materials offer benefits such as low weight, low power consumption and flexibility. Moreover, use of soluble organic materials allows use of solution processing in device manufacture, for example inkjet printing or spin-coating.
With reference to FIG. 1, an OLED may comprise a substrate 1 carrying an anode 2, a cathode 4 and an organic light-emitting layer 3 between the anode and cathode.
Holes are injected into the device through the anode 2 and electrons are injected through the cathode 4 during operation of the device. Holes in the highest occupied molecular orbital (HOMO) and electrons in the lowest unoccupied molecular orbital (LUMO) of a light-emitting material in the light-emitting layer combine to form an exciton that releases its energy as light.
Suitable light-emitting materials include small molecule, polymeric and dendrimeric materials. Suitable light-emitting polymers for use in layer 3 include poly(arylene vinylenes) such as poly(p-phenylene vinylenes) and polyarylenes such as polyfluorenes.
Polymers comprising 9,9-dialkyl substituted fluorene repeat units are disclosed in, for example, WO 99/54385.
WO 02/092723 discloses polymers comprising 9,9-diaryl substituted fluorene repeat units, which are reported to have longer lifetime that analogous polymers comprising 9,9-dialkyl substituted fluorene repeat units. This increased lifetime is attributed to an increase in thermal stability of the polymer when 9,9-dialkyl substituents are replaced with 9,9-diaryl substituents, which is manifested in higher polymer glass transition temperatures (“lifetime” as used herein means the time taken for luminance of a polymer to fall by a specified percentage, for example 10% or 50%, at constant current).
DE 19846767 discloses a 9-alkyl-9-aryl fluorene monomer.
WO 2004/039912 discloses a method of forming fluorenes with different substituents in the 9-position, such as a 9-alkyl-9-phenyl fluorenes.
WO 2009/066061 discloses a hole transport layer comprising a polymer having a repeat unit comprising a 9,9 biphenyl fluorene repeat unit wherein the 9-phenyl rings are independently and optionally substituted.